The change in legislation relating to emissions from diesel vehicles especially stipulates a significant reduction in releases of nitrogen oxide in exhaust gases. The SCR (Selective Catalytic Reduction) process in which nitrogen oxides (NOx) are reduced by ammonia is one of the technologies of choice for achieving this pollution control objective. An ammonia precursor solution (generally a solution of urea) is then injected into an exhaust line upstream of the SCR catalyst.
In a known manner, the urea solution is stored in at least one tank and it is pumped from a suction point located in one tank, then it is injected into the exhaust gases.
In the case of an SCR system comprising two tanks for storing the urea solution (for example a storage or passive tank (generally unheated) and a main or active tank (which feeds the injector and which is generally heated)), the two tanks may be connected to one another via a transfer pump that enables liquid to be transferred from the passive tank to the active tank. The system also comprises a main pump capable of pumping liquid at a suction point of the active tank and of injecting it into the exhaust gases via an injection line connected to an injector. The SCR system therefore comprises two pumps, the main pump and the transfer pump.
Such an architecture has several drawbacks: a cost premium associated with the presence of a transfer pump, recourse to supplementary electrical connections and the complexity of a control system likely not to actuate the transfer pump when the latter or the storage tank is frozen.
In the case of an SCR system comprising a single tank, the latter may have a complex architecture comprising volumes in which it is difficult to suck up the additive from a single suction point. Such an architecture is encountered, for example, in a saddle tank comprising a main pocket and a secondary pocket. The additive can then be pumped at several locations of the tank, such as for example at a main suction point located in the main pocket and at a secondary suction point located in the secondary pocket. The additive which is pumped from the main pocket by means of a main pump is then sent to the exhaust gases whilst the additive which is pumped from the secondary pocket by means of a secondary pump is conveyed to the main pocket.
In particular, the secondary suction point may be located at a low point of the tank. Such a situation may occur in architectures where the tank comprises a submerged mounting plate (as described in Application WO 2007/141312 in the name of the Applicant, the content of which is, for this purpose, incorporated in the present application) located in a “low” point of a storage tank that has a zone from which it is difficult to draw off (i.e. in fact: lower than the zone where the mounting plate is located). The additive can then be pumped from this secondary suction point via a secondary pump located, for example, in a return line and be sent to a zone of the tank different from that where the submerged mounting plate is located.
Preferably, the secondary pump is a jet pump and more preferably, a jet pump actuated by the main pump either directly (by a tapping off of its main flow for this purpose only), or indirectly (by a return line which returns to the tank the excess additive not sent to the injector). In injection mode, the additive is pumped by the main pump at a suction point in the tank, it circulates between the inlet and outlet of the pump, then it is sent into an injection line leading to an injector. The excess additive flow is sent into the return line. In such an architecture, the secondary suction point is connected directly to the jet pump through a suction line. In the return line, the excess additive pumped by the main pump primes the jet pump so that the additive located, for example, at a low point of the tank may be sucked up using the jet pump and be conveyed to the main pocket of the tank.
Preferably, the return line comprises a non-return valve located upstream of the secondary pump so that the liquid which is sucked up from the secondary pocket via the secondary pump is not recirculated to the injection line but is directly recirculated to the main pocket of the tank.
Such a system may be developed independently of the system according to the invention.